This invention relates generally to digital data receivers and specifically to a novel AGC (automatic gain control) system for HDTV (high definition television) receivers. U.S. Pat. No. 5,565,932, entitled AGC SYSTEM WITH PILOT USING DIGITAL DATA REFERENCE, issued Oct. 15, 1996 to Zenith Electronics Corporation, describes a digital television signal that is formatted in a plurality of repetitive data segments, with each data segment consisting of a fixed number of multilevel symbols occurring at a constant symbol rate of 10.76 megasymbols per second. In a receiver, the multilevel symbols are converted to corresponding digital values and the pilot, that is represented by a DC offset in the signal, is removed. The symbols are sampled, accumulated and divided to derive an average symbol value. This value is compared to a reference average symbol value to develop an AGC voltage for controlling the IF gain (and thereby the RF gain) of the receiver.
Difficulties arise in the AGC processing of rapidly changing RF signals that reach the receiving antenna. The effects of phase, delay and signal strength of dynamically ghosted signals, or of atmospheric conditions, can result in rapid changes in the total received RF signal level. A fast-acting AGC system in a receiver will tend to minimize such changes, resulting in less clipping in the analog processing circuits, particularly the IF circuits. If a signal is severely clipped, digital data will be lost and the receiver error correction circuitry will begin to fail. A fast receiver AGC will also reduce the speed requirements on the receiver equalizer, a task of which is to cancel the amplitude variations in the received signal.
Fast AGC systems are also particularly effective against zero or short delay dynamic ghosts. Such ghosts often occur in a home environment when people walk in front of an indoor antenna. In these instances, the delays are very short between the received direct and ghosted signals and the effect is much like a fast varying attenuation of a received signal. Because the tuner is designed to accommodate a large range of input signals, distortion is low, even in the presence of a dynamically changing RF input signal. Because of this, the IF AGC can be called upon to immediately track and compensate for fast moving RF signal variations and the tuner AGC can be designed to adjust its gain at a much lower rate. In the implementation of this invention, the fast AGC is applied to the IF whenever called for and is not dependent upon the slower, non-linear tuner AGC and the tuner AGC delay point.
With the invention, the IF AGC performance is optimized for speed, by employing a large capacity, continuously operated, with the most significant bits of the accumulated value being the generated AGC error correction signal. The use of the accumulator retains more resolution from the sampled data signal and the system can therefore operate in a controlled manner at a higher AGC speed. Among the benefits are: control is more precise in that errors associated with analog circuit leakage currents are eliminated; the AGC delay point is digitally stored, eliminating the need for a potentiometer; and digital averaging techniques can be applied to the detected amplitude of the recovered signal.